1. Field of the Invention
The present invention relates to a catalyst containing a catalytic trap for treating exhaust gas streams, especially those emanating from lean-burn engines, and to methods of making and using the same. More specifically, the present invention provides a catalyst containing a catalytic trap which abates NOx in the exhaust streams being treated and exhibits enhanced durability after aging at high temperature and lean operation conditions.
2. Related Art
Emission of nitrogen oxides (xe2x80x9cNOxxe2x80x9d) from lean-burn engines (described below) must be reduced in order to meet emission regulation standards. Conventional three-way conversion (xe2x80x9cTWCxe2x80x9d) automotive catalysts are suitable for abating NOx, carbon monoxide a (xe2x80x9cCOxe2x80x9d) and hydrocarbon (xe2x80x9cHCxe2x80x9d) pollutants in the exhaust of engines operated at or near stoichiometric air/fuel conditions. The precise proportion of air to fuel that results in stoichiometric conditions varies with the relative proportions of carbon and hydrogen in the fuel. An air-to-fuel (xe2x80x9cA/Fxe2x80x9d) ratio of 14.65:1 (weight of air to weight of fuel) is the stoichiometric ratio corresponding to the combustion of a hydrocarbon fuel, such as gasoline, with an average formula CH1.88. The symbol xcex is thus used to represent the result of dividing a particular A/F ratio by the stoichiometric A/F ratio for a given fuel, so that; xcex=1 is a stoichiometric mixture, xcex greater than 1 is a fuel-lean mixture and xcex less than 1 is a fuel-rich mixture.
Engines, especially gasoline-fueled engines to be used for passenger automobiles and the like, are being designed to operate under lean conditions as a fuel economy measure. Such future engines are referred to as xe2x80x9clean-burn enginesxe2x80x9d. That is, the ratio of air to fuel in the combustion mixtures supplied to such engines is maintained considerably above the stoichiometric ratio (e.g., at an air-to-fuel weight ratio of 18:1) so that the resulting exhaust gases are xe2x80x9cleanxe2x80x9d, i.e., the exhaust gases are relatively high in oxygen content. Although lean-burn engines provide enhanced fuel economy, they have the disadvantage that conventional TWC catalysts are not effective for reducing NOx emissions from such engines because of excessive oxygen in the exhaust. The prior art discloses attempts to overcome this problem by operating lean-burn engines with brief periods of fuel-rich operation (engines which operate in this fashion are sometimes referred to as xe2x80x9cpartial lean-burn enginesxe2x80x9d). It is known to treat the exhaust of such engines with a catalyst/NOx sorbent which stores NOx during periods of lean (oxygen-rich) operation, and releases the stored NOx during the rich (fuel-rich) periods of operation. During periods of rich (or stoichiometric) operation, the catalyst component of the catalyst/NOx sorbent promotes the reduction of NOx to nitrogen by reaction of NOx (including NOx released from the NOx sorbent) with HC, CO and/or hydrogen present in the exhaust.
The use of NOx storage (sorbent) components including alkaline earth metal oxides, such as oxides of Ca, Sr and Ba, alkali metal oxides such as oxides of K, Na, Li and Cs, and rare earth metal oxides such as oxides of Ce, La, Pr and Nd in combination with precious metal catalysts such as platinum dispersed on an alumina support, is known, as shown for example, at column 4, lines 19-25, of U.S. Pat. No. 5,473,887. At column 4, lines 53-57, an exemplary composition is described as containing barium (an alkaline earth metal) and a platinum catalyst. The publication Environmental Catalysts For A Better World And Life, Proceedings of the 1st World Congress at Pisa, Italy, May 1-5, 1995, published by the Societa Chimica Italiana of Rome, Italy has, at pages 45-48 of the publication, an article entitled The New Concept 3-Way Catalyst For Automotive Lean-Burn Engine Storage and Reduction Catalyst, by Takahashi et al. This article discloses the preparation of catalysts of the type described in the aforementioned U.S. Pat. No. 5,473,887 and using these catalysts for NOx purification of actual and simulated exhaust gases alternately under oxidizing (lean) and reducing (rich or stoichiometric) conditions. The conclusion is drawn in the last sentence on page 46, that NOx was stored in the catalyst under oxidizing conditions and that the stored NOx was then reduced to nitrogen under stoichiometric and reducing conditions. A similar but more detailed discussion is contained in SAE Paper 950809 published by the Society of Automotive engineers, Inc., Warrendale, Pa., and entitled Development of New Concept Three-Way Catalyst for Automotive Lean-Burn Engines, by Naoto Miyoshi et al, was delivered at the International Congress and Exposition, Detroit, Mich., Feb. 27--Mar. 2, 1995.
U.S. Pat. No. 4,742,038 discloses a metal substrate for carrying a catalytic material useful for the treatment of exhaust gases from internal combustion engines.
U.S. Pat. No. 5,874,057 discloses a method of NOx abatement utilizing a composition comprising a NOx abatement catalyst comprising platinum and, optionally, at least one other platinum group metal catalyst which is kept segregated from a NOx sorbent material. The NOx sorbent material may be one or more of oxides, carbonates, hydroxides and mixed oxides of one or more of lithium, sodium, potassium, rubidium, magnesium, calcium, strontium and barium.
Prior art catalysts as described above have a problem in practical application, particularly when the catalysts are aged by exposure to high temperatures and lean operating conditions, because after such exposure, such catalysts show a marked decrease in catalytic activity for NOx reduction, particularly at low temperature (250 to 350xc2x0 C.) and high temperature (450 to 600xc2x0 C.) operating conditions.
U.S. Pat. No. 5,451,558 discloses a catalytic material for the reduction of NOx from a turbine in a power generating stack, although the patent also refers at column 1, lines 13-14, generally to a process and apparatus for reducing pollutants xe2x80x9cwhich are produced by combustion of hydrocarbons or hydrogen in an engine or boiler, and primarily in a gas turbine.xe2x80x9d As disclosed at column 2, lines 23-37, the turbine exhaust gases are cooled to the range of 250 to 500xc2x0 F. (about 121 to 260xc2x0 C.) before contacting the catalytic/adsorbent material (column 2, lines 23-37) and the oxidation is stated (column 2, lines 45-48) to occur at temperatures in the range of 150 to about 425xc2x0 F. (66 to 218xc2x0 C.), most preferably in the range of 175 to 400xc2x0 F. (about 79 to 204xc2x0 C.). The catalytic species comprises an oxidation catalyst species which may comprise various metals including platinum group metals (see column 3, line 67 through column 4, line 3) deposited on a high surface area support which may be xe2x80x9cmade of alumina, zirconia, titania, silica or a combination of two or more of these oxides.xe2x80x9d The catalyst-containing high surface area support is coated with an adsorbent species which may comprise xe2x80x9cat least one alkali or alkaline earth compound, which can be a hydroxide compound, bicarbonate compound, or carbonate compound, or mixturesxe2x80x9d thereof. At column 3, lines 16-22, the xe2x80x9ccarbonate coatingxe2x80x9d is said to be a xe2x80x9clithium, sodium, potassium or calcium carbonate, and presently the preferred coating is a potassium carbonate.xe2x80x9d At column 4, lines 28-31, however, it is stated that the absorber comprises xe2x80x9cmost preferably sodium carbonate, potassium carbonate or calcium carbonate.xe2x80x9d The high surface area support containing the oxidation species and adsorbent may be coated onto xe2x80x9ca ceramic or metal matrix structurexe2x80x9d as a carrier, see column 4, lines 12-20.
In the ""558 patent, the catalytic material is applied to the carrier by coating the carrier with, e.g., platinum-impregnated alumina, and then wetting the alumina with an alkali or alkaline earth carbonate solution, and then drying the wetted alumina. The carrier may be alumina beads or a monolithic ceramic or stainless steel support. The use of a metal monolith support for the catalytic/adsorbent material is suggested at column 5, lines 48-58. There is no suggestion in the ""558 patent of criticality of, nor is any importance assigned to, the type of substrate or high surface area support to be used with a particular adsorbent species. In fact, as noted above, silica is one of four high surface area supports taught for use with compositions preferably including a potassium carbonate adsorbent.
WO 01014046 discloses a catalytic trap effective for conversion of NOx in an exhaust gas stream which is substantially free of silica components and includes a catalytic trap material containing a refractory metal oxide support, e.g., alumina, having dispersed thereon a catalytic component, such as a platinum group metal catalytic component, and a NOx sorbent comprised of one or more basic oxygenated compounds of potassium. The catalytic trap material is coated onto a suitable carrier member such as one made from stainless steel, titanium, alumina, titania, zirconia or silica-leached cordierite. WO 01014046 also discloses a method of treating a NOx-containing gas stream which involves maintaining the gas stream in alternating periods of lean and rich or stoichiometric conditions and contacting the gas stream with the catalytic trap under conditions in which NOx is adsorbed during periods of lean operation and released and reduced to nitrogen during periods of rich operation.
U.S. Pat. No. 5,837,212 discloses a nitrogen oxide trap comprising a porous support and catalysts consisting of manganese and potassium on the porous support. The ""212 patent indicates that by combining manganese with the potassium, it is possible to eliminate a catalytic component, such as a platinum group metal catalytic component, thereby effecting cost savings. WO 00/61289 and WO 00/67904 are to the same effect. However, the cost savings achieved by eliminating the platinum group metal component are short-lived since the resultant catalytic trap is not durable and deteriorates relatively quickly after exposure to high temperature engine operations which periodically alternate between lean and stoichiometric or rich. The initial cost savings derived by avoiding the use of precious metal components are more than overcome by the material and labor costs associated with frequent replacement of such catalytic trap.
Generally, the present invention relates to a catalytic trap member having disposed thereon a catalytic trap material comprising a catalytic component effective for the reduction of NOx and a NOx sorbent comprising an alkali metal oxide and/or alkaline earth metal oxide and a manganese component.
Specifically, in accordance with the present invention, there is provided a catalytic trap for conversion of NOx in an exhaust gas stream generated by an internal combustion engine which is periodically alternated between lean and stoichiometric or rich conditions, the catalytic trap comprising the following components: (i) a refractory metal oxide support, (ii) a catalytic component effective for promoting the reduction of NOx under stoichiometric or rich conditions of the exhaust stream and (iii) a NOx sorbent effective for adsorbing NOx under lean conditions of the exhaust gas stream and desorbing NOx under stoichiometric or rich conditions of the exhaust gas stream and comprising an alkali metal oxide and/or an alkaline earth metal oxide and a manganese component. The catalytic trap is disposed on a refractory carrier member.
The present invention pertains to a catalytic trap for abatement of NOx in an exhaust gas stream which is generated by an internal combustion engine which is operated with periodic alternations between lean and stoichiometric or rich conditions.
The catalytic trap of the invention comprises:
(A) a catalytic trap material comprising:
(i) a refractory metal oxide support;
(ii) a catalytic component effective for promoting the reduction of NOx under stoichiometric or rich conditions; and
(iii) a NOx sorbent effective for adsorbing the NOx under lean conditions and desorbing and reducing the NOx to nitrogen under stoichiometric or rich conditions, comprising:
(a) a metal oxide selected from the group consisting of alkali metal oxides, alkaline earth metal oxides and mixtures of one or more alkali metal oxides and alkaline earth metal oxides; and
(b) a manganese component selected from the group consisting of (1) a manganese oxide, (2) a mixed oxide of manganese and a transition metal and/or a rare earth metal, (3) a compound of an alkali metal and a manganese oxide, (4) a compound of an alkaline earth metal and a manganese oxide and (5) mixtures of the foregoing oxides and compounds; and
(B) a refractory carrier member on which the catalytic trap material is disposed.
The present invention also pertains to a process for abatement of NOx in an exhaust gas stream which is generated by an internal combustion engine which is operated with periodic alternations between lean and stoichiometric or rich conditions, comprising locating the above-described catalytic trap in an exhaust passage of the engine and treating the exhaust gas stream with a catalytic trap whereby at least some of the NOx in the exhaust gas stream is adsorbed by the catalytic trap during the periods of lean conditions and is desorbed from the catalytic trap and reduced to nitrogen during the periods of stoichiometric or rich conditions.
The present invention also pertains to a method for rejuvenating an xe2x80x9cagedxe2x80x9d, i.e., a spent, catalyst which contains:
(A) a catalytic trap material comprising:
(i) a refractory metal oxide support;
(ii) a catalytic component effective for promoting the reduction of NOx under stoichiometric or rich conditions; and
(iii) a NOx sorbent effective for adsorbing the NOx under lean conditions and desorbing and reducing the NOx to nitrogen under stoichiometric or rich conditions, comprising a metal oxide selected from the group consisting of alkali metal oxides, alkaline earth metal oxides and mixtures of one or more alkali metal oxides and alkaline earth metal oxides; and
(B) a refractory carrier member on which the catalytic trap material is disposed.
The spent catalyst is easily and economically rejuvenated by post-impregnating such spent catalyst with an aqueous solution of a manganese component comprising: (a) a manganese salt, e.g., manganese nitrate, acetate, sulfate, hydroxide, etc. or (b) a combination of salts of manganese and a transition metal and/or a rare earth metal or (c) a combination of salts of manganese and an alkali metal or (d) a combination of salts of manganese and an alkaline earth metal or (e) mixtures of the foregoing salts. Thereafter, the post-impregnated catalyst is dried (e.g., at 110xc2x0 C. for one hour), followed by calcination of the post-impregnated catalyst (e.g., at 550xc2x0 C. for one hour). The rejuvenated catalyst exhibits the same level of performance in abating NOx, CO and HC pollutants in the exhaust of engines operated at high temperature and lean operating conditions as exhibited by freshly prepared catalysts of the invention in which a manganese component is part of the NOx sorbent.
The refractory metal oxide support is preferably porous in nature and has a high surface area such as alumina, preferably gamma-alumina. Other suitable support materials include titania, titania-alumina, zirconia, zirconia-alumina, baria-alumina and titania-zirconia. Such support materials, useful for carrying catalyst components, are well known in the prior art. The choice of the support material is not critical to the invention. Desirably, the refractory metal oxide support will have a surface area of between about 5 and about 350 m2/g. Typically, the support will be present in the amount of about 1.5 to about 5.0 g/in3, preferably 2 to 4 g/in3.
For the purposes of the present invention the catalytic component preferably comprises a precious metal component, i.e., a platinum group metal component. Suitable precious metal components include those of platinum, palladium, rhodium and mixtures thereof. Preferably, the precious metal component comprises platinum which is present in an amount of at least about 20% by weight of the total amount of precious metal components. The catalytic component will typically be present in an amount of about 20 to about 200 g/ft3, preferably 50 to 150 g/ft3.
Component (a) of the NOx sorbent employed in the catalytic trap of the present invention comprises one or more alkali metal oxides, alkaline earth metal oxides, or a mixture of one or more alkali metal oxides and one or more alkaline earth metal oxides. Suitable alkali metal oxides include oxides of potassium, sodium, lithium, cesium and mixtures thereof. Preferably, component (a) comprises potassium oxide.
The alkali metal oxide, if present, is typically employed in an amount of about 0.05 to about 0.75 g/in3, preferably 0.1 to 0.5 in3. If component (a) of the NOx sorbent contains an alkali metal oxide, it is preferred that neither silica nor any siliceous compound be present in the catalytic trap material or in the refractory carrier member.
Suitable alkaline earth metal oxides include oxides of barium, magnesium, calcium, strontium, zinc and mixtures thereof. Preferably, the alkaline earth metal oxide comprises barium oxide. The alkaline earth metal oxide, if present, is typically employed in an amount of about 0.1 to about 3 g/in3, preferably 0.5 to 2.5 g/in3.
Component (b) of the NOx sorbent employed in the catalytic trap of the present invention comprises a manganese component which is selected from the group consisting of (1) a manganese oxide, (2) a mixed oxide of manganese and a transition metal and/or a rare earth metal, (3) a compound of an alkali metal and a manganese oxide and (4) a compound of an alkaline earth metal and a manganese oxide and (5) mixtures of the foregoing oxides and compounds. Component (b) will typically will be present in the amount of about 0.05 to about 0.5 g/in3, preferably 0.1 to 0.3 g/in3.
Suitable manganese components include (1) manganese oxides such as MnO, Mn3O4, Mn2O3, MnO2 and Mn2O7; (2) mixed oxides of manganese and transition metals and/or rare earth metals such as zirconium, titanium, tin, lanthanum, neodymium, niobium, praseodymium, etc.; (3) compounds of a manganese oxide and an alkali metal such as potassium, sodium, lithium or cesium; (4) compounds of a manganese oxide and an alkaline earth metal such as barium, magnesium, calcium, strontium, zinc, etc.
The catalytic trap of the invention is disposed on a refractory carrier member. Such substrates are well known in the prior art, e.g., stainless steel, titanium, Fecralloy, aluminum zirconate, aluminum titanate, aluminum phosphate, cordierite, mullite and corundum. The carrier member may be employed as a monolithic honeycomb structure, spun fibers, corrugated foils, layered materials, etc. The particular choice of the substrate and its configuration is not critical to the practice of the present invention.
A catalytic device employing a three-way conversion (xe2x80x9cTWCxe2x80x9d) catalyst may be used in conjunction with the catalytic trap of the invention. Such device will be located in an exhaust passage of the internal combustion engine and will be disposed upstream and/or downstream of the catalytic trap. The TWC catalyst would typically include platinum, palladium and rhodium catalytic components dispersed on a high surface area refractory support and may also contain one or more base metal oxide catalytic components such as oxides of iron, manganese or nickel. Such catalysts can be stabilized against thermal degradation by well known expedients such as impregnating an activated alumina support with one or more rare earth metal oxides, e.g., ceria. Such stabilized catalysts can sustain very high operating temperatures. For example, if a fuel cut technique is utilized, temperatures as high as 950xc2x0 C. may be sustained in the catalytic device.
If the catalytic device is employed and is located upstream of the catalytic trap of the invention, the catalytic device would be mounted close to the exhaust manifold of the engine. In such an arrangement, the TWC catalyst would warm up quickly and provide for efficient cold start emission control. Once the engine is warmed up, the TWC catalyst will remove HC, CO and NOx from the exhaust gas stream during stoichiometric or rich operation and HC and CO during lean operation. The catalytic trap of the invention would be positioned downstream of the catalytic device where the exhaust gas temperature enables maximum NOx trap efficiency. During periods of lean engine operation, when NOx passes through the TWC catalyst, NOx is stored on the catalytic trap. The catalytic trap is periodically desorbed and the NOx is reduced to nitrogen under periods of stoichiometric or rich engine operation.
If desired, a catalytic device containing a TWC catalyst may be employed downstream of the catalytic trap of the invention. Such catalytic device will serve to remove further amounts of HC and CO from the exhaust gas stream and, in particular, will provide for efficient reduction of the NOx to nitrogen under periods of stoichiometric or rich engine operation.
The several components of the catalytic trap material may be applied to the refractory carrier member, i.e., the substrate, as a mixture of two or more components or as individual components in sequential steps in a manner which will be readily apparent to those skilled in the art of catalyst manufacture. A typical method of manufacturing the catalytic trap of the present invention is to provide the catalytic trap material as a coating or layer of washcoat on the walls of the gas-flow passages of a suitable carrier member. This may be accomplished, as is well known in the art, by impregnating a fine particulate refractory metal oxide support material, e.g., gamma alumina, with one or more catalytic metal components such as a precious metal, i.e., platinum group, compound or other noble metals or base metals, drying and calcining the impregnated support particles and forming an aqueous slurry of these particles. Particles of a bulk NOx sorbent may be included in the slurry. Alternatively, the NOx sorbent may be dispersed into the support, preferably in a post-dipping operation, as described below. Activated alumina may be thermally stabilized before the catalytic components are dispersed thereon, as is well known in the art, by impregnating it with, e.g., a solution of a soluble salt of barium, lanthanum, rare earth metal or other known stabilizer precursor, and thereafter drying (e.g., at 110xc2x0 C. for one hour) and calcining (e.g., at 550xc2x0 C. for one hour) the impregnated activated alumina to form a stabilizing metal oxide dispersed onto the alumina Base metal catalysts may optionally also have been impregnated into the activated alumina, for example, by impregnating a solution of nickel nitrate into the alumina particles and calcining to provide nickel oxide dispersed in the alumina particles.
The carrier may then be immersed into the slurry of impregnated activated alumina and excess slurry removed to provide a thin coating of the slurry on the walls of the gas-flow passages of the carrier. The coated carrier is then dried and calcined to provide an adherent coating of the catalytic component and, optionally, the catalytic trap material, to the walls of the passages thereof. The carrier may then be immersed into a slurry of fine particles of component (a) of the NOx sorbent as a second or overlayer coating deposited over the layer of catalytic component. A manganese component, e.g., a solution of a manganese salt such as manganese nitrate, acetate, sulfate, hydroxide, etc., may be combined with the slurry of component (a) of the NOx sorbent or it may be applied as a third or overlayer coating deposited over the second layer of component (a) of the NOx sorbent. The carrier is then dried and calcined to provide a finished catalyst trap member in accordance with one embodiment of the present invention.
Alternatively, the alumina or other support particles impregnated with the catalytic component may be mixed with bulk or supported particles of the NOx sorbent in an aqueous slurry, and this mixed slurry of catalytic component particles and NOx sorbent particles may be applied as a coating to the walls of the gas-flow passages of the carrier. Preferably, however, for improved dispersion of the NOx sorbent, the washcoat of catalytic component material, after being dried and calcined, is immersed (post-dipped) into a solution of a component (a) of the NOx sorbent precursor compound (or complex) and a manganese precursor compound (or complex) to impregnate the washcoat with the NOx sorbent precursor. The impregnated washcoat is then dried and calcined to provide the NOx sorbent dispersed throughout the washcoat.
Separate discrete layers of washcoat may be applied in successive impregnating/drying/calcining operations, e.g., to provide a bottom washcoat layer containing a platinum catalytic component in a bottom washcoat layer and a palladium and/or rhodium catalytic component in a top washcoat layer. The NOx sorbent may be dispersed by impregnation into both the top and bottom layers.
In use, the exhaust gas stream which is contacted with the catalytic trap of the present invention is alternately adjusted between lean and stoichiometric/rich operating conditions so as to provide alternating lean operating periods and stoichiometric/rich operating periods. It will be understood that the exhaust gas stream being treated may be selectively rendered lean or stoichiometric/rich either by adjusting the air-to-fuel ratio fed to the engine generating the exhaust or by periodically injecting a reductant into the gas stream upstream of the catalytic trap. For example, the composition of the present invention is well suited to treat the exhaust of engines, including diesel engines, which continuously run lean. In such case, in order to establish a stoichiometric/rich operating period, a suitable reductant, such as fuel, may be periodically sprayed into the exhaust immediately upstream of the catalytic trap of the present invention to provide at least local (at the catalytic trap) stoichiometric/rich conditions at selected intervals. Partial lean-burn engines, such as partial lean-burn gasoline engines, are designed with controls which cause them to operate lean with brief, intermittent rich or stoichiometric conditions.
The following nonlimiting examples shall serve to illustrate the present invention and its advantages over the prior art.